Many devices are available for providing indications of temperature of surfaces. Contact devices such as thermistors or thermocouples must be placed in physical contact with the surface, a disadvantage in some applications. A further disadvantage of such temperature sensors is that they generally require an amount of time in the order of seconds to stabilize to the temperature of the surface.
Radiation detectors have been used as a noncontact alternative to such temperature sensors. Such detectors are based on the principle that the thermal radiation emitted from a surface is proportional to the temperature of the surface raised to the fourth power. Typically radiation sensors much as thermopiles respond to changes in radiation in the order of one tenth second. Unfortunately, these detectors require a clean viewing window and field of view such that the detector can see the target surface without substantial obstructions in the viewing path which can impede the accuracy of the measurement. Air purges have employed sources of compressed air and the like to drive air past the viewing window for maintaining the field of view clear of debris.
These air purges expend energy, thereby reducing the efficiency of the overall temperature detecting system. These air purges inherently require additional space and hardware, incurring further expense. Further, these purges must be periodically monitored to ensure that they are functioning properly.
It is preferable to provide a passive cleaning system that uses energy from the system, for example, from a moving target surface, to maintain the field of view of the detector substantially clean.
A method for maintaining a viewing window of a sensor substantially clean is provided which includes enclosing the sensor within a housing, and moving a target surface relative to the viewing window to create an airflow and thus a pressure drop adjacent the viewing window. That pressure drop causes air to be drawn past the viewing window of the detector. The housing can include an aperture through which the viewing window of the sensor views the target surface. Motion of the target surface creates an airflow velocity adjacent the viewing window for maintaining the viewing window substantially clean.
In one embodiment, to increase the accuracy of the detector, a high emissivity area is provided on an outside surface of the housing which faces the target surface. The high emissivity area, in one embodiment, is circular in shape and has an emissivity of greater than about 0.8. In one embodiment, the space between the target surface and the housing is limited to less than twice the diameter of high emissivity area, and preferably the space is limited to less than the diameter of the high emissivity area. As a result, radiation from the ambient area, which is not known, is substantially blocked out by the detector to increase the accuracy of the resulting temperature measurement.
In alternative embodiments, the high emissivity area can be any geometric shape. Preferably, the maximum distance between the outside surface and the target surface is less than four times, and more preferably less than two times, the distance from an optic axis of the sensor to an edge of the high emissivity area.
The sensor can include a thermopile, the cold junction of which is thermally connected to the housing.
An umbrella can be attached to an outside surface of the housing which faces the target surface for further blocking out ambient radiation. In one embodiment, the umbrella has a high emissivity area which faces the target surface. Preferably, the umbrella is thermally connected to the housing.
In one embodiment, a washer is disposed adjacent to the housing aperture, the washer having an aperture that is smaller than the housing aperture to be the limiting area in the airflow path through the housing to create a maximum airflow velocity adjacent the viewing window for maintaining the viewing window substantially clean. In alternative embodiments, the housing aperture is the limiting area in an airflow path through the housing.